Paste for formation of ceramic capacitor electrode

ABSTRACT

A paste for formation of ceramic capacitor electrode, composed of 10 to 14% by weight of an organic vehicle and 86 to 90% by weight of a platinum powder, wherein the platinum powder contains, in 100% by weight of the powder, 54 to 60% by weight of a spherical particle powder, 36 to 40% by weight of a flaky particle powder and 0 to 10% by weight of an indefinite particle powder. The electrode layer film of ceramic capacitor formed with the paste has a density, a surface roughness and an adhesion strength all of given values, can have improved adhesivity to the dielectric layer of the ceramic capacitor, and can make small the through holes generated in the electrode layer.

TECHNICAL FIELD

The present invention relates to a paste for formation of a ceramiccapacitor electrode.

BACKGROUND ART

Capacitors are an electronic part in which a dielectric layer placedbetween two electrode layers is allowed to have an electrostaticcapacity. As shown in FIG. 1, a capacitor 1 has a structure in which twoelectrode layers 3 are placed on the upper and lower surfaces of adielectric layer 2. In recent years, ceramic capacitors using a ceramicdielectric made of barium titanate (BaTiO₃) or the like have been usedin order to impart heat resistance, corrosion resistance, etc. to acapacitor.

To meet a demand for smaller electric appliances, capacitors have becomesmaller as well. Nowadays, it is a general trend that such small ceramiccapacitors use, as the electrode layer, a film formed with a paste andmoreover, in order to achieve a small size and yet a large capacity,employ a laminated structure such as shown in FIG. 2.

Conventional ceramic capacitors, however, have been insufficient inadhesivity of electrode layer film to dielectric layer; therefore, theyhave had a problem of gradual peeling of electrode layer from dielectriclayer upon use.

Peeling of electrode layer from dielectric layer results in formation ofa gap between electrode layer and dielectric layer, and an air layer lowin dielectric constant comes to be present in the gap. As a result, thecapacity of the capacitor is deteriorated with time for the abovereason, and the capacitor becomes unable to exhibit its intendedfunction.

Also, conventional ceramic capacitors have, in some cases, through holesin the electrode layer. Owing to the difference in sizes of thesethrough holes, the effective electrode area of capacitor (whichdetermines the capacity of capacitor) varies between capacitors. As aresult, the capacity of a capacitor differs greatly between capacitorsand there has been an inconvenience of low yield of capacitorproperties.

When a ceramic capacitor of laminated structure is produced, there areformed, on a dielectric layer (which later becomes one end of thelaminated capacitor produced), electrode layers and dielectric layers byturns, followed by one-time firing of the resulting laminate. At thistime, when each electrode layer interposed between two dielectric layershas through holes, the air present in the through holes and the gasgenerated during the firing expand in the holes during the firing andraise the internal pressures of the holes; as a result, interlayerpeeling between dielectric layer and electrode layer takes place.Consequently the dielectric layers give rise to blistering and cracking,and also there has been a problem of a reduction in yield of ceramiccapacitor produced. This problem can be solved by carrying out firingfor a long time; however, such firing incurs increases in running costand lead time, resulting in a higher production cost.

The present invention has been made in view of such a situation. Theobject of the present invention is to provide a paste for formation of aceramic capacitor electrode, capable of forming an electrode layer ofthe ceramic capacitor which hardly peels from the dielectric layer ofthe capacitor and giving an increased capacitor yield in production ofceramic capacitor.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a paste forformation of a ceramic capacitor electrode, composed of 10 to 14% byweight of an organic vehicle and 86 to 90% by weight of a platinumpowder, wherein the platinum powder contains, in 100% by weight of thepowder, 54 to 60% by weight of a spherical particle powder, 36 to 40% byweight of a flaky particle powder and 0 to 10% by weight of anindefinite particle powder.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a capacitor.

FIG. 2 is a schematic sectional view of a ceramic capacitor having alaminated structure.

FIGS. 3(a) and 3(b) show an example of the particle structure of aspherical particle powder. FIG. 3(a) is an enlarged photograph of 2,000magnification by secondary electron microscope observation, and FIG.3(b) is an enlarged photograph of 7,500 magnification by secondaryelectron microscope observation.

FIGS. 4(a) and 4(b) show an example of the particle structure of a flakyparticle powder. FIG. 4(a) is an enlarged photograph of 2,000magnification by secondary electron microscope observation, and FIG.4(b) is an enlarged photograph of 7,500 magnification by secondaryelectron microscope observation.

FIGS. 5(a) and 5(b) show an example of the particle structure of anindefinite particle powder. FIG. 5(a) is an enlarged photograph of 2,000magnification by secondary electron microscope observation, and FIG.5(b) is an enlarged photograph of 7,500 magnification by secondaryelectron microscope observation.

DETAILED DESCRIPTION OF THE INVENTION

The paste for formation of electrode according to the present inventionis composed of 10 to 14% by weight of an organic vehicle and 86 to 90%by weight of a platinum powder, wherein the platinum powder contains, in100% by weight of the powder, 54 to 60% by weight of a sphericalparticle powder, 36 to 40% by weight of a flaky particle powder and 0 to10% by weight of an indefinite particle powder.

The reason for specifying the content of the platinum powder in thepaste for formation of electrode, at 86 to 90% by weight, is that whenthe content is less than 86% by weight, a reduction in film densitytakes place and, when the content is more than 90% by weight,deterioration in printability takes place. More preferably, the contentof the platinum powder is 88 to 90% by weight and the content of theorganic vehicle is 10 to 12% by weight.

The reason for specifying, in 100% by weight of the platinum powder, theproportion of the spherical particle powder at 54 to 60% by weight, theproportion of the flaky particle powder at 36 to 40% by weight and theproportion of the indefinite particle powder at 0 to 10% by weight, isto allow the formed electrode layer film of the capacitor to have adensity of 80% or more, a surface roughness (Ra) of 0.4 to 0.6 μm and anadhesion strength of 2 kg or more.

When the film density is 80% or more, it is possible to suppress thedeterioration of capacitor properties and the dispersion of capacitorcapacity. Herein, “film density” is a concept indicating the compactnessof electrode film, and refers to a value [(total electrode area−holearea)/(total electrode area)] obtained by taking photographs of anelectrode surface at several different places using an electronmicroscope or a metallurgical microscope and calculating the hole areaand the total electrode area. Therefore, a larger film density indicatesthat the through holes generated in the film formed on an electrodelayer are smaller in size and number.

In the present invention, by specifying the proportions of the platinumpowders of different particle shapes in given ranges, the film densityis controlled, whereby the through holes are made small in size andnumber and the dispersion of capacitor capacity is made small. Also, thedeterioration of capacitor properties can be suppressed. Incidentally,the reason why the film density becomes 80% or more by specifying theproportions of the platinum powders of different particle shapes in theabove ranges, is that mixing of three kinds of powders different inparticle shape in a good balance allows for a higher packing density ofpowder in paste and a smaller contraction during firing.

By setting the surface roughness (Ra) of the electrode layer film at 0.4to 0.6 μm and the adhesion strength of the film at 2 kg or more, theadhesivity of the film to the dielectric layer can be enhanced and thepeeling of the electrode layer from the dielectric layer can be madesubstantially nil. By specifying the surface roughness (Ra) of the filmin the above range, the adhesivity of the film is increased with noreduction in film density; it is because in the laminated capacitorstructure, the platinum adheres to the base material by a chemicalbonding force brought about by diffusion of platinum into base materialand also by a physical bonding force brought about by an anchor effecthaving a close relation with the surface roughness. Herein, “surfaceroughness (Ra)” is a value obtained by JIS B 0601. “Adhesion strength”is a concept indicating an adhesivity between base material andconductor, and refers to a tensile strength obtained when an L-shapedlead wire of 0.6 mm in diameter consisting of a tin-coated soft copperwire is attached to a 2 mm×2 mm conductor pattern by soldering and ispulled vertically at a tensile speed of 20 mm/sec. Incidentally, byspecifying the proportions of the platinum powders of different particleshapes in the above ranges, the surface roughness (Ra) is controlled at0.4 to 0.6 μm; the reason therefor is that the flaky particle powder oflarge particle diameters constitutes the skeleton of electrode layer,the spherical particle powder and the indefinite particle powder bothhaving small particle diameters fill the gaps present between theskeleton, and thereby the surface roughness after firing is stabilized.The reason why the adhesion strength becomes 2 kg or more, is that thespherical particle powder and the indefinite particle powder both smallin particle diameter are rich in surface activity and therefore caneasily diffuse into the base material.

FIGS. 3(a) and 3(b), FIGS. 4(a) and 4(b) and FIGS. 5(a) and 5(b) showphotographs of a spherical particle powder, a flaky particle powder andan indefinite particle powder, respectively, by secondary electronmicroscope observation. The spherical particle power is a powder ofsubstantially spherical particles having diameters of submicron order;the flaky particle powder is a powder of flat particles whose majordiameters are a micron order; and the indefinite particle diameter is apowder whose primary particles have diameters of Å order, and largeparticles seen at some places are secondary particles formed byagglomeration of primary particles.

As described above, in the paste for formation of electrode according tothe present invention, the proportions of the platinum powders ofdifferent particle shapes constituting the paste are specified in givenranges; thereby, the electrode layer film made from the paste has acontrolled density, a controlled surface roughness (Ra) and a controlledadhesion strength; this makes small the dispersion of capacitor capacityand prevents the deterioration of capacitor properties.

In the platinum powder constituting the present paste for formation ofelectrode, the proportions of the spherical particle powder, the flakyparticle powder and the indefinite particle powder are more preferably57 to 60% by weight, 38 to 40% by weight and 0 to 5% by weight,respectively.

The present invention is described in more detail below by way ofExamples. However, the present invention is in no way restricted by theExamples.

EXAMPLE 1

There were compounded 12% by weight of an organic vehicle and 88% byweight of a platinum powder (which consisted of 57% by weight of aspherical particle powder, 38% by weight of a flaky particle powder and5% by weight of an indefinite particle powder), to obtain a paste forelectrode. The paste was applied to the upper and lower sides of adielectric layer to form electrode layers, followed by firing, toproduce a ceramic capacitor as shown in FIG. 1.

The lower electrode film of the capacitor had a density of 88%, asurface roughness (Ra) of 0.45 μm and an adhesion strength of 2.4 kg.

In the above ceramic capacitor, the dielectric layer was constituted bylead zirconate titanate and had a thickness of 20 μm. The electrodelayers had a thickness of 4 μm.

25 such capacitors were measured for capacity and dispersion ofcapacity. Further, they were put in continuous operation for 1,000 hoursand then measured for peeling of electrode layer from dielectric layer.The results are shown in Table 1.

EXAMPLES 2 to 9

A ceramic capacitor was produced in the same manner as in Example 1. Byusing a compounding ratio of platinum powder and organic vehicle inpaste for electrode and proportions of platinum powders of differentparticle shapes, all shown in Table 1, the lower electrode film of theceramic capacitor was allowed to have a density, a surface roughness(Ra) and an adhesion strength, all shown in Table 1.

25 such capacitors were measured for capacity and dispersion ofcapacity. Further, they were measured for peeling of electrode layerfrom dielectric layer in the same manner as in Example 1. The resultsare shown in Table 1.

Comparative Example 1

There were compounded 16% by weight of an organic vehicle and 84% byweight of the same platinum powder as used in Example 1 (which consistedof 35.7% by weight of a spherical particle powder, 40.5% by weight of aflaky particle powder and 23.8% by weight of an indefinite particlepowder), to obtain a paste for electrode. Using the paste was producedthe same ceramic capacitor as in Example 1. The lower electrode film ofthe capacitor had a density of 70%, a surface roughness (Ra) of 0.63 μmand an adhesion strength of 1.5 kg.

25 such capacitors were measured for capacity and dispersion ofcapacity. Further, they were measured for peeling of electrode layerfrom dielectric layer in the same manner as in Example 1. The resultsare shown in Table 1.

Comparative Examples 2 to 5

A ceramic capacitor was produced in the same manner as in Example 1. Byusing a compounding ratio of platinum powder and organic vehicle inpaste for electrode and proportions of platinum powders of differentparticle shapes, all shown in Table 1, the lower electrode film of theceramic capacitor was allowed to have a density, a surface roughness(Ra) and an adhesion strength, all shown in Table 1.

25 such capacitors were measured for capacity and dispersion ofcapacity. Further, they were measured for peeling of electrode layerfrom dielectric layer in the same manner as in Example 1. The resultsare shown in Table 1. As to the capacitor of Comparative Example 2, thefilm caused curling, making it impossible to measure the density of filmor the capacity of capacitor.

TABLE 1 Properties of Composition of paste for electrode (wt. %)electrode layer film Ratio of Film Surface Adhesion Capacity Plat-Organic platinum powders (%) density roughness strength AverageDispersion inum vehicle Spherical Flaky Indefinite (%) (Ra) (μm) (kg)(pF) (pF) Peeling Blistering Ex. 1 88 12 57 38 5 88 0.45 2.4 533 19 NoNo Ex. 2 88 12 60 40 0 87 0.41 2.4 527 24 No No Ex. 3 88 12 54 36 10 850.47 2.4 536 27 No No Ex. 4 90 10 60 40 0 90 0.40 2.6 550 20 No No Ex. 590 10 57 38 5 93 0.40 3.0 557 23 No No Ex. 6 90 10 54 36 10 86 0.44 2.8530 27 No No Ex. 7 86 14 60 40 0 83 0.50 2.4 520 30 No No Ex. 8 86 14 5738 5 84 0.54 2.5 519 29 No No Ex. 9 86 14 54 36 10 81 0.56 2.3 512 35 NoNo Com. Ex. 1 84 16 35.7 40.5 23.8 70 0.63 1.5 489 50 Yes Yes Com. Ex. 288 12 70 30 0 Unmeasurable 0.47 3.2 — — Yes Com. Ex. 3 86 14 35.7 40.523.8 75 0.58 1.9 494 43 No Yes Com. Ex. 4 88 12 100 0 0 76 0.20 3.5 47848 Yes Yes Com. Ex. 5 88 12 0 100 0 72 0.72 1.0 442 59 Yes Yes

As is clear from the comparison of Examples with Comparative Examples,by specifying the compounding ratio of the platinum powder and organicvehicle constituting an electrode layer film of capacitor and thedensity, surface roughness (Ra) and adhesion strength of the film, atgiven values, the peeling of the electrode layer from the dielectriclayer of the capacitor and the blistering of the dielectric layer can beprevented.

Industrial Applicability

In the paste for formation of electrode according to the presentinvention, the proportions of the platinum powders of different particleshapes contained in the paste are specified in given ranges. Therefore,in the ceramic capacitor produced using the paste, the electrode layerfilm has a density, a surface roughness (Ra) and an adhesion strengthall specified at given values; the adhesivity of the electrode layer tothe dielectric layer can be improved; and the through holes generated inthe electrode layer can be made small.

As a result, the peeling of the electrode layer from the dielectriclayer and the deterioration of capacitor properties due to the presenceof through holes can be prevented; and the yield in capacitor productioncan be improved via the suppression of dispersion of capacity.

What is claimed is:
 1. A paste for formation of a ceramic capacitorelectrode, composed of 10 to 14% by weight of an organic vehicle and 86to 90% by weight of a platinum powder, wherein the platinum powdercontains, in 100% by weight of the powder, 54 to 60% by weight of aspherical particle powder, 36 to 40% by weight of a flaky particlepowder and 0 to 10% by weight of an indefinite particle powder.
 2. Apaste for formation of a ceramic capacitor electrode according to claim1, composed of 10 to 12% by weight of the organic vehicle and 88 to 90%by weight of the platinum powder.
 3. A paste for formation of a ceramiccapacitor electrode according to claim 1, wherein the platinum powdercontains, in 100% by weight of the powder, 57 to 60% by weight of thespherical particle powder, 38 to 40% by weight of the flaky particlepowder and 0 to 5% by weight of the indefinite particle powder.
 4. Apaste for formation of a ceramic capacitor electrode according to claim1, capable of forming an electrode layer film of the ceramic capacitorhaving a density of 80% or more, a surface roughness (Ra) of 0.4 to 0.6μm and an adhesion strength of 2 kg or more.